The present invention relates to an apparatus for evaluating a vascular endothelial function in which the evaluation similar to that obtained in a measurement using an ultrasonic echo system is enabled without using an ultrasonic echo system or the like.
Recently, researches that atherosclerosis develops while showing deterioration of the vascular endothelial function as the initial phase have been conducted. In order to prevent atherosclerosis, techniques and apparatuses for evaluating the vascular endothelial function have been developed.
As a reliable technique for evaluating the vascular endothelial function, there is an apparatus called an FMD (Flow-Mediated Dilation) measurement system. In the apparatus, measurement is performed in the following manner. A cuff which is similar to that for measuring the blood pressure is attached to the arm of the subject. After occlusion of the artery is performed for a constant time of about five minutes at a pressure which is higher than the maximal blood pressure of the subject, the occlusion of the artery is released. At about three minutes after the release of the occlusion of the artery, the vessel diameter at the upstream or downstream of the cuff is measured by an ultrasonic echo system. Based on the time-dependent change rate of the vessel diameter, the vascular endothelial function is evaluated.
In the case of a normal vessel, the production of nitric monoxide (NO) which is a vasodepressor material from vascular endothelial cells is promoted by shear stress of the inner wall of the vessel due to a blood flow immediately after the occlusion of the artery. As a result, the vessel diameter is expanded. By contrast, in the case where a disorder exists in the vascular endothelial function, the degree of the expansion of the vessel diameter is decreased. When the change in vessel diameter before and after the occlusion of the artery is measured, therefore, it is possible to evaluate the vascular endothelial function.
The evaluation technique by the FMD measurement system requires skills in measurement of the vessel diameter by an ultrasonic echo system, and is difficult to handle. Furthermore, there is a problem in that the technique requires a large-scale apparatus and lacks in simplicity.
By contrast, as a technique using a simple configuration, there is a technique using a cuff pressure. In the technique, the cuff pressure is maintained at a predetermined pressure (first pressure) which is higher than the maximal blood pressure, thereafter rapidly lowered, maintained at another predetermined pressure (second pressure) which is higher than the minimal blood pressure and lower than the mean blood pressure, and, during when the cuff pressure is maintained at the other predetermined pressure (second pressure), a ratio of a cuff pressure peak value of a first pulse wave which initially appears to the maximal cuff pressure peak value which thereafter appears is calculated, thereby enabling the vascular endothelial function to be evaluated (see Patent Reference 1).
As a technique in which an index of the vascular endothelial function can be accurately measured by a simple method, there is a technique in which pressure and volume pulse waves of a vessel to be measured are measured, a ratio of variations of the pulse waves per unit time is obtained, and, with respect to the third root of the maximum value of the ratio of variations of one heartbeat cycle at rest, a ratio to a value after release of occlusion of the artery is calculated as the degree of vasodilation (Patent Reference 2).
There is another technique in which, based on the time-dependent change of posterior pulse wave information indicating a feature of the posterior half portion which is after the peak of a pulse wave reflecting variations of the vessel diameter, it is determined whether the function of vascular endothelial cells is normal or not (Patent Reference 3).
There is a further technique in which a digit probe for measuring a change of the peripheral arterial pulsatile flow is attached to a finger tip, occlusion of the artery is performed for a constant time period while attaching a cuff to the same finger tip, and a change of the peripheral arterial tone before and after the occlusion of the artery is monitored (Patent Reference 4).
In the technique disclosed in Patent Reference 1, the pressurizing periods for the pressure stimulation and the pulse wave measurement are continuous to each other. Although the pressurization for the pulse wave measurement is lower than the artery mean blood pressure, the vein blood flow is blocked, and hence the burden on the subject is large.
In the technique disclosed in Patent Reference 2, in addition to the cuff for the pressure stimulation, a sensor for measuring the volume and pressure pulse waves must be disposed. Therefore, the operation is complicated.
In the technique disclosed in Patent Reference 3, a reflected wave component which is contained in the pressure pulse wave, and which is originated from peripheral vessels is measured. Measurement of the reflected wave component and calculation of an amplitude augmentation factor AI necessitate complicated waveform recognizing and calculating processes, and an analyzing unit must have a high processing capacity.
The vascular compliance is changed by the blood pressure. When the blood pressure is high, the vessel wall is in a state where the wall is extended in the circumferential direction and hardened, and the compliance is low. Conversely, when the blood pressure is low, a force acting on the vessel wall is small. Therefore, the vessel wall is extended in a smaller degree in the circumferential direction, and the compliance is high. All of the techniques disclosed in Patent References 1, 2, and 3 have a problem in that the measured vessel information is inevitably affected by the intravascular pressure, i.e., the blood pressure.
In the technique of Patent Reference 4, a change of the peripheral arterial tone is monitored by the digit probe. In the case where amplitudes of pulse waves are compared to each other, however, the possibility that unwanted influences are included is high. Particularly, the peripheral arterial tone is caused also by the sympathetic control. Consequently, there is a problem in that the technique cannot always correctly detect the vascular endothelial function.
In view of the above-discussed circumstances, the inventors have proposed a related-art apparatus in which a cuff is wound around a part of the body, occlusion of the artery is performed for a predetermined time period by using the cuff, the pulse wave is detected by using the cuff at the same position before and after the occlusion of the artery or the like, and the detected pulse wave is analyzed to evaluate the vascular endothelial function (JP-A-2009-273870).
It has been proved that, according to the related-art apparatus, the vascular endothelial function can be adequately evaluated by using one cuff. In the related-art apparatus, the pulse wave is detected at the position where the vessel is blocked due to occlusion of the artery, ischemia due to vascular blockage therefore exerts an influence in a portion where the pulse wave is measured, and hence it is seemed that the accuracy is slightly inferior to that of a measurement using an ultrasonic echo system.
[Patent Reference 1] JP-A-2007-209492
[Patent Reference 2] JP-A-2006-181261
[Patent Reference 3] Japanese Patent No. 3,632,014
[Patent Reference 4] Japanese Patent No. 4,049,671